Radiant tube assembly

ABSTRACT

The present concept is a radiant tube assembly that includes a burner assembly firing combustion gases into a combustion tube which emits radiant heat energy from its outer surface. A black ceramic coating is applied the outer surface of the combustion tube to increase the emissivity of the combustion tube and lower the temperature of the outer surface of the combustion tube. Semi-circular top and semi-circular bottom inserts extending longitudinally along the length of a first section of the combustion tube lower the combustion tube outer temperature.

This application claims priority from U.S. provisional application 62/106,928 filed on Jan. 23, 2015 by Samer Hassan and Kevin Merritt.

FIELD OF THE INVENTION

The present invention relates to gas fired radiant heaters and more particularly relates to radiant tube assemblies for gas fired radiant heaters.

BACKGROUND OF THE INVENTION

Traditionally gas fired radiant tube heaters that are fueled with natural gas typically run at a combustion tube temperature above 750 degrees Fahrenheit and more typically closer to 900 to 1,000 degrees Fahrenheit in the hottest areas of the combustion tube normally nearest the gas burner. There are applications where these very high combustion tube temperatures cannot be tolerated for safety reasons. One such application is in repair facilities for compressed natural gas vehicles and other natural gas processing equipment. Due to the inherent danger of the presence of natural gas in repair facilities, traditionally fired natural gas fueled radiant tube assembly heaters have been unsuitable due to the very high combustion tube temperatures. Safety standards for compressed natural gas repair facilities for example dictate that the combustion tube temperature not exceed 750 degrees Fahrenheit. This is due in part to the fact that the spontaneous combustion temperature of natural gas ranges somewhere between 950 and 1,100 degrees Fahrenheit depending upon the chemical composition of the natural gas.

In order to safely utilize gas fired radiant tube heaters within compressed natural gas repair facilities it is necessary to reduce the maximum combustion tube temperature to below 750 degrees Fahrenheit in order to meet the safety standards required in these facilities.

Current gas fired radiant tube heater assemblies do not meet the requirements for use within compressed natural gas repair facilities due to the high combustion tube temperatures typically experienced with traditional gas fired radiant tube heaters.

Radiant heating however would be preferred to for example forced air heating which is currently used in many of these compressed natural gas repair facilities since there is a very high rate of air exchange due to the large doors which typically open and close numerous times per day in order to facilitate movement of vehicles in and out of buildings. Heating compressed natural gas repair facilities with conventional forced air heating is a very expensive way to maintain temperature within these buildings.

Therefore there is a need for a radiant tube heater which would meet safety standards of compressed natural gas repair facilities and other natural gas facilities which require a combustion tube temperature of no more than 750 degrees Fahrenheit in order to safely operate radiant tube heaters.

SUMMARY OF THE INVENTION

The present concept is a radiant tube assembly comprising:

-   -   a) a burner assembly firing combustion gases into a combustion         tube wherein, the combustion gases travelling down the interior         of the combustion tube;     -   b) the combustion tube emits radiant heat energy from an outer         surface of the combustion tube;     -   c) a black ceramic coating applied to an outer surface extending         from a start point and along the combustion tube for increasing         the emissivity of the combustion tube thereby lowering the         temperature of the outer surface of the combustion tube.

Preferably wherein the black ceramic coating is applied to the outer surface of the first 4 to 12 feet the combustion tube.

Preferably wherein the black ceramic coating is applied to the outer surface providing an emissivity greater than 0.88.

Preferably wherein the black ceramic coating is applied to the outer surface providing an emissivity greater than 0.90.

Preferably wherein the combustion tube includes a single top insert extending from a start point and into the combustion tube, and wherein the top insert lies adjacent to the inner surface of the combustion tube and is semi-circular in cross section wherein the insert lowers the tube outer surface temperature.

Preferably wherein at least the first 4 feet of the combustion tube includes a single top insert which lies adjacent to the inner surface of the combustion tube wherein the top insert is semi-circular in cross section wherein the insert lowers the tube outer surface temperature.

Preferably wherein at least the first 4 feet of the combustion tube includes a single top insert which lies adjacent to the inner surface of the combustion tube wherein the single top insert is over semi-circular in cross section such that it is self-supporting within the combustion tube wherein the insert lowers the tube outer surface temperature.

Preferably wherein the combustion tube includes a single top insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube and a single bottom insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube, wherein the top and bottom inserts are semi-circular in cross section and abut along a longitudinal joint wherein the inserts lower the tube outer surface temperature.

Preferably wherein at least the first 4 feet of the combustion tube includes a single top insert and a single bottom insert.

Preferably wherein the combustion tube includes a top insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube and a bottom insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube, wherein the top and bottom inserts are semi-circular in cross section and abut along a longitudinal joint and further includes an additional single top insert which abuts the other inserts at a transverse joint wherein the inserts lower the tube outer surface temperature.

Preferably wherein the single top insert is semi-circular in cross section.

Preferably wherein the single top insert is over semi-circular in cross section such that it is self-supporting within the combustion tube.

Preferably wherein at least the first 4 feet of the combustion tube includes the top and bottom inserts.

Preferably wherein at least the first 4 feet of the combustion tube includes the top and bottom inserts and the single top insert is also at least 4 feet in length.

The present concept is a radiant tube assembly comprising:

-   -   a) a burner assembly firing combustion gases into a combustion         tube wherein, the combustion gases travelling down the interior         of the combustion tube;     -   b) the combustion tube emits radiant heat energy from an outer         surface of the combustion tube;     -   c) a black ceramic coating applied to an outer surface extending         from a start point and along the combustion tube for increasing         the emissivity of the combustion tube thereby lowering the         temperature of the outer surface of the combustion tube;     -   semi-circular top insert and a semi-circular bottom insert         extending longitudinally along the length of a first section of         the combustion tube, the inserts for lowering the combustion         tube outer temperature.

Preferably wherein inserts extend along the length of the first two sections of the combustion tube.

Preferably wherein the first section extends from a start point to 3 to 6 feet along the length of the combustion tube.

Preferably wherein the second section extends from 6 to 12 feet along the length of the combustion tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The present concept will be described by way of example only with reference to the following drawings in which:

FIG. 1 is a schematic side cross sectional view of a radiant tube assembly with a burner assembly and a combustion tube.

FIG. 2 is a schematic cross sectional view taken along lines 2-2 of FIG. 1 showing the cross section through the combustion tube with the top and bottom inserts in place.

FIG. 3 is a cross sectional schematic view of the combustion tube taken along lines 3-3 showing a single top insert in place.

FIG. 4 is a schematic perspective view of the radiant tube assembly together with a burner assembly and a combustion tube.

FIG. 5 is a plot of temperature vs. length along the combustion tube for a 100,000 BTU burner showing top temperatures.

FIG. 6 is a plot of temperature vs. length along the combustion tube for a 100,000 BTU burner showing top temperatures.

FIG. 7 is a plot of temperature vs. length along the combustion tube for an 80,000 BTU burner showing top temperatures.

FIG. 8 is a plot of temperature vs. length along the combustion tube for an 80,000 BTU burner showing bottom temperatures.

FIG. 9 is a plot of temperature vs. length along the combustion tube for a 60,000 BTU burner showing top and bottom temperatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present concept a radiant tube assembly shown generally as 100 includes a combustion tube 102 and a burner assembly 104. Burner assembly 104 mixes air and fuel together to produce combustion gases 107 which travel down combustion tube 102 thereby heating combustion tube 102 which emits heat energy in the form of radiant energy from outer surface 125.

Combustion tube 102 includes a first section 106 and a second section 108. First section 106 includes a top insert 110 and a bottom insert 112 which join at a longitudinal joint 114 along the length of each of the inserts. Top insert 110 abuts with bottom insert 112 along the longitudinal joint line 114. Second section 108 includes a single top insert 116 which abuts and joins at transverse joint line 118 with top and bottom insert 110 and 112. The balance of the combustion tube 102 normally would not include any further inserts.

First section 106 may be three to six feet in length and second section 108 may also be typically three to six feet in length. It was found that the two top inserts depicted could be combined into one of between 4 to 12 feet in length, rather than two shorter top inserts. In some cases 4 feet of top insert was sufficient depending upon the burner firing rate. In practice however it was more convenient and installation was easier by dividing the top inserts into two lengths as depicted of about three to six feet each.

The outer surface 125 of combustion tube 102 is coated with a ceramic exterior coating 120 to improve the emissivity of the outer surface 125 of combustion tube 102 thereby lowering the temperature of the outer surface 125 of the combustion tube 102. Increasing emissivity results in more energy being released by the combustion tube 102 thereby lowering its outer surface temperature at a given firing rate. This is contrary to conventional thinking which attempts to maximize outer surface 125 tube temperatures. Preferably a black ceramic coating is applied to the outer surface having an emissivity>0.88 and preferably>0.90. In practice black ceramic coatings yield emissivity values typically from 0.90 to 0.93.

At minimum the first four feet of the combustion tube is coated however as much as 12 feet may be coasted. On average it was found that the first six to ten feet need to be coated to obtain optimum results. For reference purposes combustion tube 102 includes a top 122 and a bottom 124 and a start point 128.

Referring now to FIGS. 2 and 3 the reader will note that the inserts namely top insert 110 and bottom insert 112 and single top insert 116 are inserted into the inner diameter of combustion tube 102 and lay adjacent to the inner surface 127 of combustion tube 102. Similarly single top insert 116 normally extends partially around the inner surface 127 of combustion tube 102 and past the center line 129 as depicted in FIG. 3 in order to maintain the positioning of single top insert 116. This is termed over semi-cylindrical in shape.

Top insert 110 is preferably semicircular in cross section namely a semi-cylinder such that the top half of the inner surface 127 of the combustion tube is lined. Top insert 110 which is a semi-cylindrical is depicted in FIGS. 1 and 2. In the event there is only a top insert and no bottom insert the single top insert 116 may have a cross section which is over semi-circular such that more than the top half of the inner surface 127 of the combustion tube is lined with a top insert which is an over semi-cylinder as depicted in FIGS. 1 and 3. In this manner the single top insert 116 is self-supporting within the combustion tube 102. In theory a semi-cylindrical shaped insert should be self-supporting however in practice an over semi-cylindrical shaped insert is required to obtain consistent self support. The bottom insert 112 is normally semi-cylindrical since it is normally used in conjunction with a top inset 110. FIG. 4 shows in schematic fashion radiant tube assembly 100 with a combustion tube 102 and the burner assembly 104.

Referring now to FIGS. 5 through 9 all of which are graphs which plot distance measured in feet on the x axis against outer surface tube temperature in degrees Fahrenheit on the Y axis.

The starting point or zero in feet is start point 128 which normally is approximately 3 inches in from the end of burner assembly 104. Temperatures are measured on the top 122 and the bottom 124 of the outer surface of combustion tube 102 at one foot intervals by thermocouples arranged on the top and bottom surfaces of combustion tube 102 in order to measure the temperatures.

The reader will see that graph 5 for example depicts the temperature in degrees Fahrenheit along the top of combustion tube in one foot intervals under various conditions.

For example the thick solid black line shows a conventional gas fired radiant tube heater with no exterior coating 120 on the outer surface 125 and only with a top insert 110 and a single top insert 116 in place. You will see that the maximum temperature achieved is over 950 degrees Fahrenheit at approximately the three foot mark along the length of the combustion tube 102.

The long dashed line shows the temperature profile with a black ceramic exterior coating 120 on the outer surface 125 and once again two inserts in place namely top insert 110 and single top insert 116 in this case the maximum temperature achieved is approximately 740 degrees Fahrenheit at approximately four feet along the combustion tube.

The third line which is the short dashed line depicts the result of a black ceramic exterior coating 120 on the outer surface 125 of combustion tube 102 together with three inserts in place a top insert 110 a bottom insert 112 and a single top insert 116.

The reader will note that the outer surface combustion tube maximum temperature achieved in this case is approximately 710 degrees Fahrenheit at about 8 feet along the combustion tube 102.

Similarly in FIG. 6 the three graphs are depicted for temperatures which are measured along the bottom 124 of combustion tube 102 again at one foot intervals.

FIGS. 5 and 6 are for a burner rated at a 100,000 BTU firing rate.

FIGS. 7 and 8 are for an 80,000 BTU firing rate burner wherein FIG. 7 depicts the temperature profile on the top 122 for black ceramic exterior coatings 120 in place on the combustion tube together with two inserts namely top insert 110 and single top insert 116 and three inserts namely top and bottom inserts 110 and 112 and a single top 116.

FIG. 9 depicts the temperature profiled for a 60,000 BTU firing rate for a combustion tube 102 which has a black ceramic exterior coating 120 and only two inserts in place namely top insert 110 and single top insert 116. The dark black line shows the top temperatures taken along top 122 and the dashed line shows bottom temperatures taken along the bottom 124.

In Use

The reader will note that the greatest temperature reduction is achieved by the use of the black exterior ceramic coating 120 along the outer surface 125 of combustion tube 102. The use of the coating itself however achieves the results which were marginal in terms of reducing the overall temperature to below 750 degrees Fahrenheit on an ongoing basis.

In addition to the exterior coating 120 inserts were added to the interior of combustion tube 102 namely top insert 110, bottom insert 112 and single top insert 116 along the first two sections namely first section 106 and second section 108 as depicted in FIG. 1 in order to further reduce the combustion to a temperature as shown in the short dashed line.

The reader will note that in particular referring to FIG. 6 the bottom insert 112 had a dramatic effect in reducing the bottom 124 temperatures to well below 750 degrees Fahrenheit which was not the case in for example the large dashed line showing a coated combustion tube with only 2 inserts namely top insert 110 and single top insert 116 in place.

The reader will note that in every case the inventors were able to reduce the maximum temperature seen along the top of 122 and bottom 124 of the combustion tube 102 to below 750 degrees Fahrenheit with a combination of either exterior ceramic coating 120 together with combustion tube top insert 110, bottom insert 112 and single top insert 116 as required.

It should be apparent to persons skilled in the arts that various modifications and adaptation of this structure described above are possible without departure from the spirit of the invention the scope of which defined in the appended claim. 

I claim:
 1. A radiant tube assembly comprising: a) a burner assembly firing combustion gases into a combustion tube wherein, the combustion gases travelling down the interior of the combustion tube; b) the combustion tube emits radiant heat energy from an outer surface of the combustion tube; c) a black ceramic coating applied to an outer surface extending from a start point and along the combustion tube for increasing the emissivity of the combustion tube thereby lowering the temperature of the outer surface of the combustion tube.
 2. The radiant tube assembly claimed in claim 1 wherein the black ceramic coating is applied to the outer surface of the first 4 to 12 feet of the combustion tube
 3. The radiant tube assembly claimed in claim 2 wherein the black ceramic coating is applied to the outer surface providing an emissivity greater than 0.88.
 4. The radiant tube assembly claimed in claim 2 wherein the black ceramic coating is applied to the outer surface providing an emissivity greater than 0.90.
 5. The radiant tube assembly claimed in claim 3 wherein the combustion tube includes a single top insert extending from a start point and into the combustion tube, and wherein the top insert lies adjacent to the inner surface of the combustion tube and is semi-circular in cross section wherein the insert lowers the tube outer surface temperature.
 6. The radiant tube assembly claimed in claim 3 wherein at least the first 4 feet of the combustion tube includes a single top insert which lies adjacent to the inner surface of the combustion tube wherein the top insert is semi-circular in cross section wherein the insert lowers the tube outer surface temperature.
 7. The radiant tube assembly claimed in claim 3 wherein at least the first 4 feet of the combustion tube includes a single top insert which lies adjacent to the inner surface of the combustion tube wherein the single top insert is over semi-circular in cross section such that it is self-supporting within the combustion tube wherein the insert lowers the tube outer surface temperature.
 8. The radiant tube assembly claimed in claim 3 wherein the combustion tube includes a single top insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube and a single bottom insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube, wherein the top and bottom inserts are semi-circular in cross section and abut along a longitudinal joint wherein the inserts lower the tube outer surface temperature.
 9. The radiant tube assembly claimed in claim 8 wherein at least the first 4 feet of the combustion tube includes a single top insert and a single bottom insert.
 10. The radiant tube assembly claimed in claim 3 wherein the combustion tube includes a top insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube and a bottom insert extending from a start point and into the combustion tube, which lies adjacent to the inner surface of the combustion tube, wherein the top and bottom inserts are semi-circular in cross section and abut along a longitudinal joint and further includes an additional single top insert which abuts the other inserts at a transverse joint wherein the inserts lower the tube outer surface temperature.
 11. The radiant tube assembly claimed in claim 10 wherein the single top insert is semi-circular in cross section.
 12. The radiant tube assembly claimed in claim 10 wherein the single top insert is over semi-circular in cross section such that it is self-supporting within the combustion tube.
 13. The radiant tube assembly claimed in claim 10 wherein at least the first 4 feet of the combustion tube includes the top and bottom inserts.
 14. The radiant tube assembly claimed in claim 10 wherein at least the first 4 feet of the combustion tube includes the top and bottom inserts and the single top insert is also at least 4 feet in length.
 15. A radiant tube assembly comprising: a) a burner assembly firing combustion gases into a combustion tube wherein, the combustion gases travelling down the interior of the combustion tube; b) the combustion tube emits radiant heat energy from an outer surface of the combustion tube; c) a black ceramic coating applied to an outer surface extending from a start point and along the combustion tube for increasing the emissivity of the combustion tube thereby lowering the temperature of the outer surface of the combustion tube; d) a semi-circular top insert and a semi-circular bottom insert extending longitudinally along the length of a first section of the combustion tube, the inserts for lowering the combustion tube outer temperature.
 16. The radiant tube assembly claimed in claim 15 wherein inserts extend along the length of the first two sections of the combustion tube.
 17. The radiant tube assembly claimed in claim 15 wherein the first section extends from a start point to 3 to 6 feet along the length of the combustion tube.
 18. The radiant tube assembly claimed in claim 16 wherein the second section extends from 6 to 12 feet along the length of the combustion tube. 